I. Field of the Invention
The invention is related to multi-resistance element assemblies, and in particular to the multi-resistance element assemblies used to vary the speed of small D.C. motors.
II. Description of the Prior Art
Often it is desirable to have the speed of small D.C. motors variable to accommodate different conditions. For example, it is desirable to operate the windshield wiper of an automotive vehicle at a fast speed during a heavy rain, and at a lower speed during a light rain or drizzle. Likewise, it is desirable to operate a defroster, heater, or airconditioning fan of an automotive vehicle at various speeds for comparable reasons. Presently, there are several ways to vary the speeds of these motors. One is to use multiple field windings within the motor itself, such as used in two speed windshield wiper motors of present day vehicles. This requires additional field windings within the motor itself increasing its cost. Because of cost considerations, the method is usually limited to two speeds. For motors requiring less power, such as fan motors, the prior art teaches placing or switching one or more resistances in series with the motor to change the motor's speed. Because the current requirements of the small D.C. motors are still relatively high, the prior art uses coils of resistance wire such as nichrome or similar alloys, having the desired resistance. Although the fabrication of the individual components is relatively inexpensive, a typical three-resistance element assembly will require at least seven (7) individual parts which have to be fastened together. With the high cost of labor, the labor for the final assembly of the multi-resistance assembly represents the majority of its cost. The disclosed invention is a resistance element assembly which comprises only two parts which may be snapped together without the use of special tools or fixtures, thereby providing a significant reduction of labor costs.
The key to the invention is an expanded metal resistance element as taught by S. W. Melson in U.S. Pat. No. 1,991,935. Melson teaches the use of the expanded metal as a resistive element for small motors or similr devices in which the expanded metal is formed into a tubular shape. Bolt holes are formed in the unexpanded portion for mounting and electrical connection purposes. For multiple resistance element devices, Melson further teaches slitting or cutting a slit through the non-expanded portion and its contiguous expanded portion from one end, then cutting a second slit through the non-expanded portion and its contiguous portion from the other end. Multiple slits may be cut in an alternating sequence to form the desired number of resistance elements. The unslit non-expanded portions serially connect adjacent expanded resistive elements. The expanded portions are then bent into tubular form and the unexpanded portions formed into mounting tabs. The final assembly comprises a bolt and a number of insulators and spacers which hold the resistive elements spaced from each other without deformation. Another type of resistive element is taught by Schaefer in U.S. Pat. No. 2,422,542, in which a resistive woven wire grid is soldered to electrical terminals. The use of expanded metals in resistive heating devices is taught by Horsfall et al. in U.S. Pat. No. 3,543,970, Maake in U.S. Pat. Nos. 3,798,419 and 3,860,789, Seel in U.S. Pat. No. 3,835,435, Schladitz in U.S. Pat. No. 3,843,950, and Ballard in U.S. Pat. No. 4,100,395.